In a decorative item, for example, an exterior part (member) of a timepiece, such as a wristwatch band, bezel, casing, back lid, buckle or dial, use is made of stainless steel, titanium or a titanium alloy. In particular, austenitic stainless steel which is excellent in corrosion resistance and ornamental capacity is widely employed as the stainless steel.
For example, a plate of austenitic stainless steel SUS 316 or SUS 304 is subjected to cold forging. The forged plate is arbitrarily cut or drilled (punched) and finished into the shape of wristwatch band piece. The thus obtained band pieces are connected to each other to obtain a completed wristwatch band.
However, the austenitic stainless steel has a drawback in that its specular surface is easily scratched to cause the appearance of the wristwatch band, bezel, casing, back lid, dial or other exterior part of a timepiece constituted by the austenitic stainless steel per se to easily deteriorate.
The technology of carburizing the surface of stainless steel such as austenitic stainless steel to harden the stainless steel surface is now being studied in order to resolve the above drawback. However, the carburized stainless steel surface suffers from generation of a strain in the crystal lattice of stainless steel because of the penetration of carbon atoms to become a rough surface. Thus, also, the technology of further polishing the carburized stainless steel surface into a specular surface is being investigated.
For example, in Japanese Patent Laid-open Publication No. 54(1979)-86441, it is described that a specular surface can be easily obtained by subjecting fine precision parts, such as a gear, a spring and a shaft, constituted by a low carbon steel, a low alloy case hardening steel or the like, although there is no description of austenitic stainless steel, to pack carburizing (solid carburizing) at 900° C. and thereafter subjecting the surface of such parts to barrel polishing.
However, when a metal having a high content of chromium, such as austenitic stainless steel, is carburized at high temperatures such as 700° C. or above, chromium carbide is precipitated in a surface portion of stainless steel. As a result, the chromium content of stainless steel per se is reduced to cause the corrosion resistance of the stainless steel to extremely deteriorate. Further, the chromium carbide becomes bulky, thereby posing such a problem that the carburized region of stainless steel cannot have high hardness.
The method of carburizing austenitic stainless steel at low temperatures such as less than 700° C. for the purpose of avoiding the above precipitation of chromium carbide can be contemplated. However, when the carburization is conducted at such low temperatures, a passive film which hinders the penetration of carbon atoms is formed on the surface of stainless steel to thereby disenable hardening of the stainless steel surface.
In recent years, the technology of hardening a surface of austenitic stainless steel while maintaining the excellent corrosion resistance of the austenitic stainless steel is being investigated. For example, in the technology disclosed in Japanese Patent Laid-open Publication Nos. 9(1997)-71854, 9(1997)-268364 and 9(1997)-302456, austenitic stainless steel is fluorinated in a fluorogas atmosphere at low temperatures such as 300 to 500° C. to convert the above passive film to a fluorinated film through which carbon atoms can be easily penetrated. The fluorinated austenitic stainless steel is sequentially subjected to gas carburizing in a carburizing gas atmosphere at low temperatures such as 400 to 500° C. and to pickling or mechanical polishing (for example, soft blasting, barrel polishing or buffing).
In the thus-obtained decorative item, for example, a wristwatch band, a hardened layer, i.e., carburized layer extending from a surface of austenitic stainless steel as a base material to a depth of 5 to 50 μm is formed while maintaining the corrosion resistance thereof. Therefore, not only does the base material exhibit a beautiful specular surface but also the specular surface has a Vickers hardness (HV) as high as 500 to 700, which cannot be attained by stainless steel provided with no surface hardening treatment.
The decorative item (including personal ornament) constituted by the austenitic stainless steel having its surface hardened is resistant to scratching, so that there is an advantage such that the beauty thereof can be maintained for a prolonged period of time.
However, even if the decorative item whose base material is constituted by the above stainless steel having its surface hardened sometimes suffers from scratching when a sharp intense external force is applied thereto.
Therefore, there is a demand for the development of a decorative item whose base material is constituted by stainless steel having a higher surface hardness, i.e., greater scratch resistance than that of the conventional decorative item, and also for the development of a process for producing such a decorative item.
On the other hand, with respect to the decorative item such as an exterior part of wristwatch or a bracelet, it, as a personal ornament, must have the same ornamental value as those of other decorative items. Accordingly, the personal ornament at its surface is often furnished with an ornamental coating. For example, a gold alloy coating formed by wet plating is widely employed as such an ornamental coating.
However, the gold alloy coating is soft and easily scratched. Accordingly, even if the hardened base material surface of a personal ornament is covered with the soft gold alloy coating, the gold alloy coating would be scratched, spoiling its beauty as a decorative item. This personal ornament has a drawback in that the above advantage of hardening of the base material surface cannot be utilized.
Therefore, there is a demand for the development of a decorative item whose surface hardness as measured from the surface of gold alloy coating is large even if the gold alloy coating per se is soft, that is, a decorative item which is excellent in scratch resistance, and for the development of a process for producing the same.
In the technology described in Japanese Patent Laid-open Publication Nos. 9(1997)-71854, 9(1997)268364 and 9(1997)-302456, austenitic stainless steel is carburized at low temperature, so that precipitation and bulking of chromium carbide in stainless steel would not occur. However, a layer wherein, mainly, Fe and C in stainless steel are simultaneously present, possibly “mill scale (roll scale, black scale)” containing an iron oxide such as Fe2O3, is formed on an outermost surface of carburized layer. In the technology described in the above literature, the mill scale is removed by pickling or mechanical polishing.
However, with respect to the exterior part of a timepiece constituted by stainless steel which has been gas carburized at low temperature as mentioned above, completely removing the mill scale formed on the surface thereof so as to render the exterior part of the surface specular cannot be accomplished only by performing mechanical polishing such as barrel polishing or buffing. The reason is that most timepiece exterior parts have a complex configuration because of the attainment of ornamental beauty with the result that there are places which cannot be polished, such as the inside wall of holes and inside the wall and bottom of recessed portions. Further, with respect to timepiece exterior parts comprising a plurality of parts connected to each other, it is difficult to polish part interfaces. For example, with respect to a wristwatch band comprising a multiplicity of band pieces connected to each other by means of connecting parts, the smaller the interstice of mutually neighboring band pieces, the more difficult the polishing thereof.
Moreover, the surface of timepiece exterior parts cannot also be rendered specular only by pickling. In the pickling described in the above literature, iron contained in the mill scale is leached with a strong acid solution to remove the mill scale from the surface of timepiece exterior parts. However, iron is also contained in stainless steel per se, so that the surface of carburized layer is corroded by the strong acid solution. As a result, the surface of carburized layer after pickling is roughened and cannot be specular.
Furthermore, finishing to be effected on the surface of timepiece exterior parts constituted by stainless steel is not limited to specular finishing. For realizing an ornamental beauty, various mechanical finishings are required. For example, hairline finishing wherein a multiplicity of mutually parallel nicks are engraved or honing wherein a multiplicity of fine recesses are engraved is required.
However, the carburized stainless steel has a drawback in that its surface is so hard that it is difficult to effect the above mechanical finishing thereon.
Therefore, there is a demand for the development of an exterior part of a timepiece constituted by stainless steel such as austenitic stainless steel which is excellent in scratch resistance and has a specular surface without detriment to the inherent excellent corrosion resistance of stainless steel; the development of an exterior part of the timepiece constituted by stainless steel such as austenitic stainless steel which is excellent in scratch resistance and has its surface provided with mechanical finishing such as hairline finishing or honing without detriment to the inherent excellent corrosion resistance of stainless steel; and the development of a process for producing such timepiece exterior parts.
The timepiece exterior parts, for example, band pieces gas carburized at low temperatures such as 400 to 500° C. as mentioned above are not furnished with a beautiful specular surface required for exterior ornamentation of timepieces, and the surface thereof is observed as “orange peel” having fine unevennesses, despite the implementation of polishing.
The reason is that, by the gas carburization, a greater amount of carbon is diffused within the metal crystal grains of the stainless steel surface than in the metal crystal grain boundaries. That is, when carbon penetrates the metal crystal grains, the metal crystal grains become bulky and swell outward with the result that a thickness difference occurs between the crystal grains and the crystal grain boundaries. When viewed from the surface of the stainless steel, the crystal grains are higher than the crystal grain boundaries.
The above height difference between the crystal grains and the crystal grain boundaries cannot be eliminated despite the implementation of a sequence of treatments after the gas carburization, including pickling and mechanical polishing. As a result, the crystal grains are likely to be recognized as being lifted from the surface of stainless steel, and a multiplicity of lifted crystal grains are observed as fine unevennesses of stainless steel surface, i.e., “orange peel”.
This “orange peel” is a phenomenon which commonly occurs when not only stainless steel but also titanium, a titanium alloy or other metals for use in exterior ornamentation of timepieces are subjected to surface hardening, for example, carburizing at a temperature which is close to the recrystallization temperature of the metal or below. In particular, the orange peel is a phenomenon which occurs when surface hardening is performed at below a temperature slightly over the recrystallization temperature of the metal.
Further, this “orange peel” is not limited to carburization wherein carbon is used as a solute atom, and is a phenomenon which commonly occurs when surface hardening is performed with the use of nitrogen or oxygen as a solute atom at temperature which is close to the recrystallization temperature of the employed metal or below.
Accordingly, there is a demand for the development of an exterior part of a timepiece with an excellent appearance, constituted by a metal which has a smooth or specular surface free of “orange peel” even if the metal is subjected to surface hardening at a temperature which is close to the recrystallization temperature of the metal or below; and for the development of a process for producing such an exterior part of the timepiece.
It is an object of the present invention to solve the above problems of the prior art and to provide a decorative item comprising a base material having a hardened layer, for example, a carburized layer extending from a surface thereof to an arbitrary depth, the base material surface having a higher surface hardness, i.e., greater scratch resistance than that of the conventional decorative item, especially an exterior part of a timepiece with such a characteristic.
It is another object of the present invention to provide a decorative item comprising the above base material with a hardened layer, the decorative item having a surface furnished with a golden color or other various tones without any lowering of surface hardness, i.e., without detriment to the scratch resistance thereof, especially an exterior part of the timepiece with such a characteristic.
It is an additional object of the present invention to provide processes for producing the above decorative items.
Further objects of the present invention are to provide an exterior part of a timepiece constituted by stainless steel such as austenitic stainless steel which is excellent in scratch resistance and has a specular surface without detriment to the inherent excellent corrosion resistance of stainless steel; to provide an exterior part of a timepiece constituted by stainless steel such as austenitic stainless steel which is excellent in scratch resistance and has its surface provided with mechanical finishing such as hairline finishing or honing without detriment to the inherent excellent corrosion resistance of stainless steel; and to provide a process for producing such timepiece exterior parts.
It is a still further object of the present invention to provide an exterior part of a timepiece with an excellent appearance, constituted by a metal which has a smooth or specular surface free of “orange peel” even if the metal is subjected to surface hardening at a temperature which is close to the recrystallization temperature of the metal or below; and to provide a process for producing such an exterior part of a timepiece.